Cooling and heat pump systems with rotary compressors, primarily those having means for oil injection, normally include an oil separator located on the high pressure side of the system. This means that an oil store is subjected to the highest pressure of the system and by a suitable choice of the oil channel system, oil which is utilized for lubrication, cooling and sealing in the compressor may be caused to circulate by means of the "natural" pressure difference of the cooling system. Cooling and heat pump systems operate under very different operating conditions and also under such operating conditions in which this pressure difference is too small for a sufficient oil circulation. This may be the case, for example, in booster (low pressure) operation with certain cooling agents, but this may also occur, for example, during longer start up periods in systems having large condenser and/or evaporator volumes in relation to the capacity of the compressor. In these cases the oil circulation can be ensured by the connection of an oil pump. However, in addition to the intended properties of maintaining the pressure difference and the circulation, such an oil pump has certain negative properties, such as restricting the oil flow to the given pump capacity and constituting a loss source since it requires power for its operation. The restriction of the oil flow implies that it is necessary to choose a sufficiently large pump for the most unfavourable operating case, which may involve too great a pump capacity for the normal operation case, resulting in an unnecessarily great driving power.
The aim must be that the oil pump, if such is required, should not be larger than what is necessary with regard to the most unfavourable operating case and only be utilized where necessary. This objective can be fulfilled in many different ways, for example by means of shunting and valves with forced operation, which normally involves expensive solutions.